1. Field of the Invention
The present invention relates to a so-called COFDM (“Coded Orthogonal Frequency Multiplex”) demodulator or analogue.
2. Discussion of the Related Art
FIG. 1 is intended to illustrate the principle of a COFDM modulation. Data packets to be transmitted are put in the form of N complex coefficients associated with N respective frequencies (or carriers). Number N of the frequencies is equal, for example, to 1705 for the so-called “2K” mode and to 6817 for the so-called “8K” mode, in digital television hertzian transmission. Each complex coefficient corresponds to a vector which is illustrated in FIG. 1 as starting from a frequency axis at a point indicating the frequency associated with the coefficient.
These N coefficients are processed by inverse fast Fourier transform (IFFT), which generates a “symbol” formed of a sum of modulated carriers, each carrier having an amplitude and a phase determined by the associated complex coefficient. The symbol thus generated is transmitted.
Conventionally, in hertzian transmission, the width of the transmission channel is of 6, 7, or 8 MHz and each carrier is separated from the next one by a frequency interval Δf=1/Tu. Tu is the duration of transmission of a symbol and is called the useful duration. The useful duration is on the order of 224 μs in mode 2K and of 896 μs in mode 8K, for a bandwidth of 8 MHz.
Upon reception, a receiver submits the symbol to the inverse processing, that is, mainly a fast Fourier transform (FFT) to restore the original complex coefficients.
As shown in FIG. 1, some regularly distributed vectors P1, P2, P3 have a known constant value. These vectors, or the corresponding carriers, are called pilots. They are used to reflect the distortions undergone by the transmitted signal and, by the information that they provide on the channel response, they enable correcting the unknown vectors located between the pilots.
FIG. 2 illustrates a transmission of several successive symbols Sn−1, Sn . . . As show, each of these symbols is preceded by a guard interval Tgn−1, Tgn, which is a copy of part of the end of the corresponding symbol. The guard intervals are often defined by a fraction of useful duration Tu. Conventional values of the guard interval are Tu/32, Tu/16, Tu/8, or Tu/4.
The guard intervals are used to avoid inter-symbol modulation distortions caused by an echo of the transmission. FIG. 2 also shows an echo SEn−1, TgEn−1 . . . of the transmitted signal. This echo is delayed with respect to the main signal by a duration smaller than that of a guard interval Tg.
Each symbol S is normally analyzed by the FFT circuit of the receiver in a window W of same length as the symbol. If there was no guard interval, an analysis window W coinciding with a symbol of the main signal would be astride two symbols of the echo signal. This would cause an error difficult to correct in the FFT calculation.
Guard interval Tg, provided that it is greater than the echo delay or advance, provides a margin for the setting of analysis window W so that it only coincides with portions belonging to the same symbol, in the main signal as well as in the echo. The fact for an analysis window to be astride a symbol and its guard interval introduces a phase shift which is corrected by means of the above-mentioned pilots.
In FIG. 2, symbol Sn−1 must be analyzed in a window Wn−1 that can extend from time ta, corresponding to the beginning of the guard interval of echo TgEn−1, to time tb corresponding to the end of symbol Sn−1. Similarly, symbol Sn must be analyzed in a window Wn that can extend from time tc, corresponding to the beginning of the guard interval of echo TgEn, to time td corresponding to the end of symbol Sn.
The positioning of analysis windows W is conventionally performed by means of pilots contained in the symbols.
FIG. 3 schematically shows the place of pilots in the symbols. The symbols are gathered in frames of 68 symbols, conventional in digital television hertzian transmission (standard ETSI EN 300 744, V1.4.1).
In FIG. 3, each line shows a symbol and each square represents the position of a carrier. The carriers are defined as going from a position 0 to a position Kmax, Kmax being equal to 1,704 in mode 2K and to 6,816 in mode 8K. Indeed, only a portion of the possible frequencies is used, especially due to risks of losses at the channel borders. There are two types of pilots.
On the one hand, there are, in each symbol, continuous pilots Pc. The continuous pilots correspond to specific frequencies distributed in the channel. In above-mentioned standard ETSI, there are 45 pilots in mode 2K and 177 pilots in mode 8K. The continuous pilots are present in all symbols and always occupy the same frequency position. In FIG. 3, only the continuous pilots corresponding to positions 0 and Kmax have been shown.
On the other hand, there are, in each symbol, so-called “scattered” pilots Pr, which are arranged every 12 carriers, and shifted by three positions between two successive symbols. Thus, every four symbols, the same arrangement of scattered pilots Pr is found.
The continuous and scattered pilots, of constant amplitude upon transmission, are used to know the pulse response of the channel and, accordingly, in most known circuits, finely position analysis window W.
For this purpose, at the receiver, the complex received time signal, after having been put in baseband, is provided to a fast Fourier transform unit providing the symbol in the frequency field. The pilots are sampled from this symbol. The sampled pilots enable frequency estimation of the channel which, after having undergone an inverse Fourier transform, provides the channel pulse response. The channel pulse response is used to finely position the window.
A problem exists due to the fact that the pilots only provide an incomplete description of the channel. Indeed, taking into account several successive symbols (at least four) only enables having an image of the channel every three points. As a result, the estimation of the pulse response of the channel obtained from the pilots exhibits a periodization of a period equal to useful duration Tu divided by three. Accordingly, if guard interval Tg is relatively large as compared to the useful duration, it is no longer possible to optimally position the window. Such is the case when Tg is equal to Tu/4. An intersymbol modulation, often accompanied by a data loss, may then occur.
A solution to this problem is to analyze the data received after complete decoding and to determine their bit error rate BER. The window location is then modified by trial and error until the bit error rate is acceptable. A problem of this way of proceeding is its inaccuracy and its slowness. Indeed, the complete decoding of the data must be awaited, which may be very long. Generally, with the above solution, a data loss almost inevitably occurs.